We studied the interaction involved in Gal repression using the analytical ultracentrifuge. The repression loop apparently involves binding of GalR to the galactose operator sites on the DNA but for suppression to occur the presence of HU has been found to be necessary. We studied first the self-association properties of HU and GalR separately to establish the patterns and strengths of the self-associations. This was followed by the study of the binding of GalR to the oligonucleotide containing one of the two galactose operators (a 29 base-pair oligonucleotide). Finally HU was added and using the previously established association patterns and their association strengths we deduced that a pair of HU monomers binds to the complex of the oligonucleotide with a GalR dimer. The association strength was also established and the results were consistent with those obtained by energy transfer fluorescence and polarization anisotropy measurements. The results have been submitted to J Biol Chem.